This invention relates to charge-injection devices, and more specifically to charge-injection device arrays for image sensing.
Presently, large area-high resolution charge-injection device (CID) arrays are fabricated for image sensing applications. The charge injection devices which make up these arrays are basically metal-insulator-semiconductor (MIS) diodes biased to produce a depletion well at the surface of the device. Semi-transparent (thin) metal electrodes are used so that radiation may pass through the electrodes and into the semiconductor where the radiation is absorbed by band-to-band excitation and produces minority carriers which are collected and stored in the potential well.
The CID array is constructed so that the row electrodes of the devices in a row are connected to a common point and that the column electrodes of the devices in a column are connected to a common point. To read the charge collected in a device, a potential is applied to the row electrode of the selected device. The output voltage of the column in which the device is located is then clamped to a set level. An injection pulse is then applied to the column output line, and the charge on each device in the column, except for that of the device in the row addressed, is transferred to the row well. The charge in the column well of the addressed device is injected into the substrate by the injection pulse. The change in voltage on the column output after the injection pulse has passed is the signal assigned to the addressed device.
Such an array is described by Burke and Michon in IEEE Transactions on Electron Devices, Vol. ED-23, no. 1, February 1976, and discusses the operation and performance of CID arrays. The current method of fabrication and information retrieval from CID arrays yields real-time image data, but problems do exist for this application. One problem is that the column outputs in an array may have different dc levels. This is due primarily to time constant effects in the array, and the recovery from the injection pulse which saturates the pre-amp and amplifier circuits. Thermal drift may also cause varying column outputs.
Another problem is that of "blackening" or non-charged devices appearing to be black when in a column with charged devices. This problem occurs because the charge on the output line is changing between the clamp and sample of the read operation, due to the continuing charge integration in the non-addressed device. Anagnostospoulos, in IEEE Transactions on Electron Devices, Vol. ED-25, No. 1, February 1978, attempts a solution to the problem of blackening, which he refers to as "cross-talk". His solution involves special circuitry on the column output lines which drastically reduces the sensitivity of the device.
It is an object of the present invention to provide a CID array which is fabricated with the goal of providing solutions to the problems of blackening and column output level variations. It is also an object of the invention to provide a new technique for information retrieval from the new CID array, as part of the solution to the previously stated problems.